This invention relates to methods for monitoring an artificial lift oil well produced by sucker rod pumping, and more particularly, to pump-off controllers.
Most artificial lift wells are produced by sucker rod pumping, most commonly with a beam pumping system. In these systems, a surface prime mover acting through a gear reducer powers reciprocation of a sucker rod string. The sucker rod string is attached to a subsurface plunger that reciprocates within a working barrel which either is integrally connected to the bottom of the well tubing or is integrally part of a subsurface pump assembly packed off against the tubing (or casing where tubing is not installed). The plunger has an aperture that is opened and closed by a "traveling" valve. In the clearance space below the bottom reach of the plunger, the head of the working barrel has an intake aperture that is opened and closed by a "standing" valve. In general, the column of oil fluids in the tubing (or casing) is supported by the working barrel head when the traveling valve is opened and the standing valve is closed, and by the rod string and plunger when the traveling valve is closed.
In an ordinary pump, at the start of the rod-drawn plunger upstroke the traveling valve closes, and the fluid column load is picked up by the rods. As the plunger moves up, flued in the pump chamber clearance space expands and pressure within the chamber decreases to the pump intake pressure at which the standing valve opens, whereupon fluid from the producing zone enters the pump chamber. As the rods and plunger continue their upstroke, the fluid column above the plunger is lifted essentially by the distance of upstroke travel, and a displaced volume of fluid essentially equal to the swept volume of the plunger in the working barrel is collected at the surface. During this upstroke, the pump chamber fills with producing zone fluids. On reaching the top of the upstroke and starting the downstroke, the standing valve closes and the traveling valve, under the weight of the undisplaced fluid column, remains closed. Gas (if present) in the pump chamber is compressed until pressure in the chamber increases to the pump discharge pressure at which the traveling valve opens, and fluid load is transferred from the rods to the tubing. As the rods and plunger continue their downstroke, fluids within the chamber are displaced up through the traveling valve aperture into the tubing.
If the producing zone pressure is insufficient to cause complete liquid fillage of the pump chamber during the upstroke of the plunger, the traveling valve does not open on the ensuing downstroke until the plunger approaches and encounters the relatively incompressible liquid in the chamber. The resulting "impact" between the plunger and the liquid produces an upward force, and the "load" on the plunger is released suddenly. This causes a pounding, called "fluid pound", that can be damaging to the rod string, the pump assembly and the surface pumping unit. When this condition of incomplete pump chamber fillage happens, the well is said to "pump off". Aside from possible damage caused by fluid pound, operating a pumping unit when incomplete pump chamber fillage is occurring is wasteful of power relative to fluids produced, since volumetric efficiency of the pump is lower.
Devices called pump-off controllers have been developed to sense when pump-off occurs, so that the surface pumping unit can be shut down to reduce possible mechanical damage to the equipment and eliminate wasteful use of power. After a preset period of shut off, the pumping unit is then restarted. Many pump-off controllers are equipped with a mechanical malfunction shut down feature used to detect parted rods and inoperative pumps. Run time totalizers may also be employed, to indicate a worn pump or tubing leaks, or changes in well conditions such as well decline and water flood response.
Pump-off controllers generally are of two types, local logic and central computer control. The local logic type is a self contained system mounted at the pumping unit. Investment cost is comparatively low, but the system must be monitored and adjusted manually at the well site. Central computer control involves sensors installed on the pumping equipment. Data from the sensors are transmitted by cable or other telemetry to a central computer for well monitoring and control. Investment cost is relatively high, but the system has the advantage of being able to monitor wells at a central point to minimize down time caused by malfunctions.
Pump-off controllers differ in the methods or techniques of sensing pump-off. The more widely used methods of sensing pump-off are: polished rod load, motor current, vibration, flow/no flow, and bottom hole producing pressure.
Currently the most common method of sensing pump-off is monitoring polished rod load. Polished rod load monitoring techniques can be broken down into three categories: rod work, rate of change of load on the downstroke, and rod load at a particular polished rod position on the downstroke. My invention disclosed in U.S. Pat. No. 3,951,209 measures polished rod load and displacement and integrates these measures numerically to obtain power input to the polished rod and rod string at the surface. Because the power required at the downhole pump decreases when the well pumps off, pump-off is indicated by a reduction in the power input to the rod string at the surface.
Rate of change of load on the downstroke can usually be used to detect pump-off, because a fluid pound is often associated with a rapid load change on the downstroke. However, a fluid pound at the pump is not always clearly defined at the surface because of rod stretch and dynamics, and these conditions can make the load rate of change concept less sensitive to pump-off.
Another variation uses rod load to a position in the upper portion of the downstroke. This is sampled under a filling condition and is used as a reference. When a fluid pound occurs, rod load departs from the reference load and pump-off is sensed. An example is U.S. Pat. No. 4,286,925. This method of detecting pump-off is difficult to adjust and maintain, and a position marker switch must be used.
Controllers which use polished rod monitoring techniques require position and/or load transducers and, where digital computers are involved, associated analog to digital converters.
Motor current is widely used to sense changes in polished rod loads and changes in polished rod work, hence pump-off, since the product of the current and voltage is roughly proportional to polished rod work and voltage is nearly constant. As pump fillage changes from complete to partial, the upstroke current peak changes only slightly; however the downstroke current peak can change appreciably. This is because the fluid load remains on the rods during the downstroke until the traveling valve is opened. As a result the unit often becomes more rod heavy when pump fillage is reduced. The rod heavy condition causes the upstroke current peak to change relative to the downstroke current peak.
Examples of patents involving a motor current method for detecting pump-off are U.S. Pat. Nos. 3,363,573; 3,953,777 and 3,998,568. In practice, the most widely used techniques employ motor current averaging. When a well is pumped off and pounding, less current is required by the electric motor and consequently the average current for the stroke reciprocation cycle is less than when complete pump fillage is occurring; thus a decrease in average current levels is used to sense pump-off. However, available controllers which use the motor current averaging method do not adequately differentiate between generating currents and motoring currents. As may be seen by reference to the current curve illustrated in FIG. 1, it is seen that motor current decreases with increasing speeds of revolution of the motor until the synchronous speed of the motor is reached; at speeds greater than the synchronous speed, motor current increases. The motor's operating current in the rotational speed range from starting to synchronous speed is known as the motoring current, and the operating current in the speed range which is greater than the synchronous speed is known as the generating current. Since current increases when synchronous speed is exceeded, but also as the motor labors harder below synchronous speed, motor load cannot be simply related to average motor current, and this is believed to be a major cause of unsatisfactory performance of these pump-off controllers.
Other techniques using motor current sense a difference in motor current peaks or sense current at a point on the downstroke. To use a difference in current peaks, the controller requires the unit to be in balance or slightly rod heavy, otherwise the controller logic can be confused. Using current at a point on the downstroke is difficult to calibrate and to maintain in adjustment, and requires a position marker.
The vibration method of sensing pump-off operates on the principle that a shock load or vibration is usually associated with a fluid pound. A sensor is installed on the unit structure, normally the walking beam. When the load or vibration increases in magnitude to the shock load setting of the sensor, fluid pound is sensed and the unit is shut down. Examples of this method are U.S. Pat. Nos. 2,661,697 and 3,851,995. However, a fluid pound at the pump is not always evident at the surface, especially in deep wells that are operating at a slow pumping speed, and under these conditions, the vibration sensing method is not especially effective.
In the flow/no flow method, a flow rate sensor is placed in the flow line. When the well pumps off, the producing rate is reduced. The sensor is calibrated to sense the reduction in pumping rate over a preselected period of time. If the rate is below a preset threshold, pump-off is determined and the unit is shut down. Examples of a flow/no flow method are U.S. Pat. Nos. 2,550,093; 2,697,984; and 3,105,443. In general, the flow/no flow method is difficult to adjust and can be confused by well heading.
In the bottom hole producing pressure method, a pressure sensor is used to measure the bottom hole producing pressure. Pressure data are transmitted by electric cable to the surface controller. When the producing pressure is reduced to a preset amount, the unit is shut down and restarted after an adjustable time delay. This is a good method of controlling pump-off, but has the disadvantage of high initial costs and high maintenance costs. Problems associated with the data transmission cable are common.